Just an FYI for ya...
It is not technically possible to get pregnant from anal sex; there is no way for semen to get from the rectal tract to the vaginal tract.
However, anal sex is still not a very good method of birth control. Semen leaking from the anus after intercourse may drip across the perineum (the short stretch of skin separating vulva and anus) and cause what is known as a 'splash' conception. The failure rate for this is surprisingly high! 8% of couples of who use anal sex as a method of birth control have babies each year.
http://www.minou.com/aboutsex/analsex.htm